Mold or dirt
Our 20 year old home has fg bats in the basement ceiling (kraft backed, krafty paper out)that run into the rim joists. I have been slowly pulling back the insulation, adding 1.5" XPS to the joise and sealing up tight and then pushing the fg back in place.
Now and then I pull a bat down and its dusty / dirty, sometimes there is black dusty stuff usually oalong the edge of the bat. I can not tell if its mold or dirt. I do know that where the kraft paper meets the insulation is solid black in color and I wonder somehow thats not coming off after 20+ years.
We have never seen any water or had moisture issue. The basement is generally around 50% hummidity and is being slowly finished. I have never seen this black stuff on any wood or anywhere else.
I could post a picture if you think that would help. I think I have seen this stuff in areas other than the rim joist areas too.
I am not really sure what mold looks like...this stuff does not appear to be alive...it almost looks like coal dust or dark black dirt in the fg, again usually by the edge. Thoughts?